Tunable magnetron



Unite States Patent TUNABLE MAGNETRON Paul W. Crapuchettes, Palo Alto,Calif., assignor to Litton Industries, Inc.

Application November 29, 1952, Serial No. 323,269

4 Claims. (Cl. SIS-39.61)

This invention relates to magnetrons of the multieavity resonator typeand particularly to magnetrons which are tunable.

Within recent years cavity magnetrons have been developed in which theelectrical constants of the cavities could be changed, therebypermitting the tuning of magnetrons over a range of frequencies. Theresonant frequency of the magnetron can be changed by varying theresonant circuits of the anode in some manner. Several methods are used,either singly or in combination, to tune magnetrons; one of whichconsists in employing a C ring which adds capacitance between the strapsof a ring-strap magnetron to lower the resonant frequency of the mode byan amount, depending upon the position of the C ring. Another methodemploys a ring termed an L ring that is placed over the ends of thecavities and raises the resonant frequency as it is moved closer to theanode block by interfering with the alternating magnetic field thatpasses through the cavity. A detailed explanation of a combination L-Cring may be found in the U. S. patent entitled Adjustable Magnetron, No.

2,512,901, filed November 1, 1945, and issued June 27,

1950. Another approach to the tuning problem consists in insertingconducting rods in the cavities parallel to the axis of the cathode.Such rods reduce the effective crosssectional area of the cavity,thereby reducing magnetic flux and increasing the resonant frequency.Another method for tuning'consists of a single pin or plunger which isinsertable into one of the cavities to change the characteristicsthereof, and since the resonant cavities are closely coupled to eachother, it is possible in this way to modify the resonant frequency ofthe magnetron. A shortcoming of this latter method is that the tuningrange is limited because the characteristics of only one cavity isaffected.

It is an object of this invention to provide a simple and unique tuningmeans which permits tuning over a greater range than comparable tuningmeans heretofore known.

In accordance with an aspect of this invention, there is provided atunable magnetron comprising a supporting body and a plurality of vanesmounted inside the body integrally therewith; the size and position ofthe vanes forming together with the body a plurality of resonatorsdetermining substantially the mean frequency to which the magnetron maybe tuned. The magnetron is tuned by a conductive member having a tuningportion positioned to enter into or egress from two or more resonators.

A highly desirable feature of this invention is the provision of tuningmeans which simultaneously modifies the electrical characteristics ofthree resonant cavities, thus greatly improving the tuning range overthe pin and plunger type tuners, which can modify the characteristics ofonly a single cavity.

The above mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will be best understood, by reference to the following descrip-Patented Sept. 16, 1958 mounted on its guiding structure and themechanism for' adjusting the same to the desired position; and

Fig. 2 is a cross-sectional view looking toward the plane perpendicularto the axis of symmetry of Fig. 1. The figure shows essentially theconstruction and positioning of the tuning element forming the basis ofthis invention.

Since the general structure of the magnetron is not part of my presentinvention, but is described in the aforementioned patent, it will onlybe briefly described here. Referring to the figures, the referencecharacter 1 represents the body of the magnetron to which other partsare brazed or otherwise suitably fastened to forma structure capable ofbeing evacuated. On a cylindrical section 2 is mounted the cathodesupport assembly comprising the necessary members to properly supportthe cathode 3 and to supply the cathode with suitable electrical energy.The cathode may preferably comprise two filament windings of thoriatedtungsten so mounted and energized as to constitute a bifilar filament.One end of the filament is shown connected to a power supply lead 4.Pole pieces 5, 6 are provided to supply thenecessary magnetic fluxparallel to the axis of the cathode in the well known manner.

Oscillator energy from the magnetron is extracted for useful work bycoupling a waveguide 7 to one of the' cavities through an impedancetransformer. The transformer may comprise a pair of ramps 8 positionedwithin a horn shaped member 9. Adjacent to the cavities, the ramps areclose together and, therefore, of low imped-. ance to substantiallymatch the impedance of the cavities. The ramps diverge outwardly and asthey approach the waveguide 7 their separation and the internaldimensions of the horn determines their impedance which is designed tosubstantially match that of the waveguide. The principles underlyingthis method of coupling energy from the magnetron may be found in theabove mentioned patent.

The resonating cavities of the magnetron are formed by brazing aplurality of vanes 10 around the inner diameter of the body 1 as shown.Each cavity may constitute, in effect, a quarter wave line which whenresonating has a magnetic flux concentration adjacent its shorted end,the short being the inner wall of the body 1. As a is well known, whenthe magnetron is in operation, the instantaneous operating potentials ofthe adjacent vanes which form the cavities are equal and opposite. Thatis, the currents in adjacent cavities are of opposite phase and thecurrents in alternate cavities are in like phase. To closely couple thealternate cavities together and to cause them tooscillate in effect as asingle cavity, the alternate vanes are fastened together in a knownmanner by conductive straps 11 and 12 as shown in Figures 1 and 2.

The magnetron structure as described above constitutes a knownstructure, the details ofwhich may be found in the above-mentionedpatent. In accordance with the present invention there is provided, inaddition to the above defined structure, a novel tuning means whichmodifies the electrical characteristics of a plurality of cavshapedmember. However, the tuning portion of the paddle member has a curvedforward wall corresponding to the curvature of the magnetron wall andthe end walls of the paddle member are angularly disposed parallel withrespect to the adjacent complete vanes. Thus, it is seen that thegeometrical discontinuity is maintained at a minimum during the tuningoperation and practically nonexistent when the paddle member iswithdrawn so that the wall thereof is aligned with the wall of themagnetron.

A platform 17 is sealed to the body portion 1 and has a receptacle 18for accommodating the supporting member 15. The movement of the paddletuning member 14 within the cavity does not affect the evacuatedcondition of the resonator cavities by virtue of a Sylphon bellows 19being hermetically sealed at one end 20 to the platform '17, and at theother end 21 to the cup-shaped member 16. A rod 22, which is movablerotatably but not longitudinally, threadedly engages the interior of theinner portion 23 of the cup member 16. Thus, when the rod 22 is rotated,the cup member 16, supporting member 15 and paddle tuning member 14 moveunitarily in the longitudinal direction. That is, into or out of thecavity. The upper portion of the rod 22 threadedly engages a shouldermember 24 against which a plurality of washers 25 act to seal tightlythe hollow portion 26 of the cylin-v drical housing 13. An end member 27closes the housing 13 and is positioned by a thread engagement with thetubular portion of the housing 13. A set screw 28 may be used to preventthe rod 22 from rotation.

A plurality of the vanes 10, defining the cavity resonator of themagnetron, have a cut out portion 29 (Fig. 2), whereby the tuner paddle14 may enter into or egress from without contacting the sides of thevanes. By way of illustration, two vanes are provided with cut outportions, thereby permitting tuning of three cavities simultaneously. Itis important to recognize that the clearance between the paddle memberand the cut out in the vanes is critical. If this clearance is made verysmall, then the tuner can be made to tune more than 500 me. at 9000 me.As the paddle tuner is moved into the cavity, it blocks off the normalpath of flux, thus reducing the inductance of the cavity and, raisingthe frequency. Therefore, if leakage between the paddle member and cutout is great, a greatly lessened tuning range will result. For bestoperation the clearance should be as small as a width greater than oneresonator cavity width, means possible, while maintaining separationbetween the paddle tuner and resonator vanes.

While I have described above the principles of my invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention as setforth in the objects thereof and inthe accompanying claims.

What is claimed:

1. A tunable magnetron comprising a supporting body, a pluralityof vanesmounted inside said body integrally therewith forming a plurality ofresonators, the size and position of said vanes determiningsubstantially a normal frequency at which the magnetron may operate, andmeans mounted on said body for tuning said magnetron over a givenfrequency range, said means characterized by a single conductive memberhaving a tuning portion of for mounting said member for linear radialmovement to enter into and egress from at least two adjacent resonatorsat an intermediate point on a longitudinal wall of said resonator body,and means for moving said member to tune said magnetron.v f

2. A magnetron according to claim 1, wherein said plurality of vanesconstituting the tuning resonators have a cut out portion along onelongitudinal edge thereof, and said tuning portion being adapted toenter into and egress from the cut out portions of said vanes.

3. A magnetron according to claim 1, wherein said tuning portion of theconductive member .comprises a forward wall having a curvaturecorresponding to the radius of curvature of said body and angularlydisposed end walls parallel to adjacent vanes, whereby the geometricaldiscontinuity is maintained at a during the movement of said tuningmember.

4. The magnetron according to claim 2, wherein the thickness of thetuning portion is only slightly less than the cut out portion of saidvanes.

References Cited in the file of this patent UNITED STATES PATENTS

